The various components of gas mixtures can be separated from each other by flowing the mixture in contact with a separation medium. The separation medium may comprise a selectively adsorbent material and/or a filter material. The separation medium is disposed in a vessel through which a feed gas mixture is flowed, and the separation medium selectively adsorbs or blocks one component of the mixture and allows another component to pass through. As a result, the outflow product gas from the vessel has a lower proportion of the adsorbed or blocked gas than the feed gas mixture. For example, when the feed gas mixture is air and the separation medium selectively adsorbs oxygen, the outflow product gas is nitrogen-rich relative to the feed gas mixture. The separation process is typically carried out at an elevated gas pressure inside the vessel. When an adsorbent separation medium is “spent,” i.e., when the adsorbent material has adsorbed all that it can adsorb, the adsorbent material can be recharged by desorbing and removing the adsorbed gas. In pressure swing adsorption (PSA) systems, the desorption process includes lowering the pressure inside the vessel to facilitate desorption of the adsorbed gas molecules.
The adsorbent selectivity of a separation medium is determined at least in part the size of the pores in the separation medium. Accordingly, gas molecules with a kinetic diameter less than or equal to the pore size of the separation medium are retained, or adsorbed, on or by the separation medium while gas molecules of larger diameters pass through the separation medium. The separation medium, in effect, sieves the gas according to its molecular size and is therefore sometimes referred to as a molecular sieve. A molecular sieve for the production of nitrogen-rich gas from air should have a pore structure with a diameter comparable to the kinetic diameter of oxygen. Conversely, some zeolite molecular sieve materials are known for selectively adsorbing nitrogen from air. Accordingly, nitrogen is adsorbed by the zeolite molecular sieve while oxygen passes through. Carbonaceous materials are also known for use as adsorbents (e.g., activated carbon) for adsorbing carbon species such as oil vapors from air to produce clean air as an output product gas, and as molecular sieve materials (Carbon Molecular Sieves (CMS)) for nitrogen generation from air.
As seen in FIG. 1, a prior art vessel 100 for use in generating nitrogen gas from air in a PSA process is configured as an upright oblong cylinder 102 having rounded end portions 104, 106 welded thereto, with a feed gas inlet 108 at the top rounded end and a product gas outlet 110 at the bottom rounded end. There is a bottom bed support screen 114 near the bottom of the vessel 100, at the weld line for the bottom rounded end portion 104, and a top bed screen 116 near the top of the vessel, at the weld line for the top rounded end portion 106. A gas flow path through the vessel, indicated by arrows 120, proceeds axially in a single, downward direction from the feed gas inlet to the product gas outlet. Such vessels are typically more than six feet tall and provide bed spaces that have large length-to-diameter (L:D) ratios (i.e., L:D greater than 4:1)(based on a length LPA (along the central axis APA of the cylinder) from the bottom bed support screen 114 to the top bed screen 116, which corresponds to the length of the cylinder portion, excluding the rounded ends, and a specific diameter DPA). A prior art single bed vessel 100 is configured to have a L:D ratio of greater than 4:1 because such a configuration is believed to favor gas permeation through the bed of separation medium therein and to prevent “channeling,” i.e., to prevent the flow of gas around the bed or through channels in the bed as opposed to permeation through the bed.